INFLUENCE OF INCUBATION-PERIOD HUMIDITY ON THE DEVELOPMENT OF BROWN-ROT BLOSSOM BLIGHT OF SOUR CHERRY

When detached sour cherry (Prunus cerasus) blossoms were inoculated wi th conidia of Monilinia fructicola and subjected to a standard 8-h wet ting treatment at 20 degrees C, blossom blight incidence was proportio nal to relative humidity (RH) when RH was held constant during the sub sequent 6-day incubation period (frequency = 1.0 at the maximum RH of 92%; frequency = 0.38 at the minimum RH of 57%). Similarly, when a pri mary incubation period at 87% RH was followed by a secondary incubatio n period at 54% RH, blossom blight incidence was proportional to the n umber of hours at the higher level (frequencies of 0.94, 0.80, and 0.3 8 with primary incubation periods of 6 days, 36 h, and 12 h, respectiv ely). When intact blossoms on potted trees were exposed to common inoc ulation and wetting treatments, disease incidence was consistently hig h on trees that subsequently were incubated in a controlled environmen t chamber (20 degrees C, 90 to 95% RH) but was extremely variable when trees were incubated under variable ambient conditions. Ambient incub ation temperature had little effect on disease incidence 9 days after inoculation, whereas ambient RH had a pronounced effect: the frequency of blighted blossoms was 0.53 to 0.61 when the number of hours at RH >90% was approximately two to six times that at RH <60%, whereas this frequency was only 0.02 to 0.07 when the number of hours at RH >90% wa s approximately one-third the number at RH <60%. After 48 h at a const ant RH of 89 or 57%, the water potential of excised uninoculated bloss oms was -1.15 and -1.93 MPa, respectively; however, growth of M. fruct icola on osmotically adjusted potato dextrose agar was unaffected by c hanges in water potential within this range. Thus, although RH during incubation has an important influence on blossom blight development, t he causal mechanism remains uncertain.